1. Field of the Invention
The present invention relates to intelligent devices that are in communication with one another over one or more physical media, without needing a network controller.
2. Description of the Related Art
Communication among low-cost devices is useful in many applications. For example, in a home environment, room occupancy sensors, light switches, lamp dimmers, and a gateway to the Internet can all work together if they are in communication. A room in a home could be illuminated when people are present, or else an alarm could be sounded, depending on conditions established by a program running on a remote computer.
Networks of communicating devices can be constructed using many different physical communications media. Fiber optics, coaxial cable, twisted pair, and other structured wiring are commonly used for relatively high-performance local-area networks of PCs, printers and other computing devices. Infrared signals can be used at short range within a single room, as with handheld remote controls. Cell phones, pagers, and long-distance microwave communications use licensed radio-frequency (RF) bands.
When used to create networks of low-cost devices in existing buildings, such as a home, these physical media all have drawbacks. Structured wiring is not present in most homes already constructed, and adding it is disruptive and expensive. Infrared signals do not cover the whole house. Licensed RF normally requires payment of a subscription fee by users.
Existing powerline wiring and unlicensed RF bands do not have these problems. They are ubiquitous and can be freely used by anyone. They do, however, have other problems. Devices on the powerline must communicate despite the presence of substantial noise, attenuation by equipment powered by the line, and possible interference among devices. Devices using unlicensed RF bands must emit low power, and unlicensed bands are often crowded with multiple users.
Many existing devices that communicate on the powerline use a protocol known as X10 (see U.S. Pat. No. 4,200,862, dated Apr. 29, 1980). Signaling is accomplished using bursts of low-amplitude 120 KHz cycles synchronized to the powerline zero-crossings. Most X10 devices can either transmit or receive, but not both. X10 transmitters send commands without expecting a confirmation reply, so most control is open-loop. Users must typically configure an X10 system by manually setting mechanical “house code” and “unit code” switches on receiving devices.
With the addition of RF-to-X10 translation devices, commands originating in RF handheld remote controls can operate X10 receivers on the powerline. RF repeater devices can be used to increase the range of such a system. The RF remote control signaling protocols are typically independent of the X10 signaling protocol, and are more akin to signaling protocols used by infrared remote controls.
Improvements to basic X10 devices are available in the marketplace (see Smarthomepro Catalog, 63D, Spring 2004). Two-way communications, involving confirmation of commands and retries when necessary, enable much more reliable performance. Repeaters and phase bridges can solve signal attenuation problems. Use of nonvolatile storage, such as EEPROM, along with system controllers, can simplify system setup.
Powerline and RF signaling using spread-spectrum technology can be faster and more reliable than narrowband X10-like signaling. U.S. Pat. No. 5,090,024, dated Feb. 18, 1992, discloses such a system, and U.S. Pat. No. 5,777,544, dated Jul. 7, 1998, discloses spread spectrum in combination with narrowband signaling. Encoding and decoding of spread-spectrum signals is more complex than narrowband processing, and more complexity means higher relative cost.
Regardless of the method used for signaling on the physical media, systems comprising multiple devices must deal with the problem of mutual interference. Many solutions to this problem are in common usage.
In frequency-division multiplexing (FDM), narrowband devices hold conversations by tuning to different bands within the available spectrum. Spread-spectrum systems can use different spreading sequences for code-division multiplexing (CDM), or they can use different frequency-hopping patterns. Using these methods, each device in a network must know specifically how to connect with every other device. Broadcasting to all devices at once is not possible.
It is less complex, and more common, for all devices to share the same communications channel. Narrowband communication on the powerline or within an RF band is less costly than frequency-agile methods. Device interference is typically managed using time-division multiplexing (TDM) or carrier-sense multiple-access (CSMA) methods.
TDM allows each device on a medium to transmit data only within particular timeslots. Timeslots can be assigned in various ways.
IEEE 802.5 Token Ring networks dynamically assign timeslots to devices that wish to transmit by passing a special packet called a token. A device may transmit only when it possesses the token. After transmission is complete, the device releases the token to the next device.
U.S. Pat. Nos. 5,838,226, 5,848,054, 5,905,442, and 6,687,487, assigned to Lutron Electronics Company, disclose a lighting control system comprising masters, slaves and repeaters that communicate via RF and also on the powerline.
In particular, a number of RF repeater devices can be installed in a system to enhance signaling reliability. To avoid the problem of repeaters interfering with each other, the Lutron patents require an installer to set up one repeater as a main repeater, and others as second, third, and subsequent repeaters. This designation corresponds to a strict timeslot assignment for each repeater, such that a given repeater is only permitted to transmit during its respective preassigned timeslot. Furthermore, multiple slave devices in the system can transmit their status back to a master controller. These slave devices also use timeslots established during an installation procedure for each device.
CSMA networks are based on a listen-before-talk rule. Talkers break up messages into short packets. Talkers then listen for the medium to be quiet before sending a packet. Even so, data collisions can still occur, so CSMA-CD-CR (collision-detect, collision resolution) methods are often implemented. Collision-detection might involve closed-loop communications using special acknowledge/non-acknowledge (ACK/NAK) packets sent by packet addressees. Collision resolution might involve a transmission retry after a random delay.
The object of both TDM and CSMA systems is to allow only one device to use the available communications channel at any given time. Therefore, the signal strength on the channel depends on which single device is currently transmitting.